Grenmarite, a new mineral of the götzenite-seidozerite-rosenbuschite group, was found in a syenite pegmatite at the eastern side of the island Vesle Arøya in the Langesundsfjord district, Norway. Grenmarite, ideally (Zr,Mn)2(Zr,Ti)(Mn,Na) (Na,Ca)4(Si2O7)2(O,F)4, is the Zr-analogue of seidozerite. It is monoclinic with space group P2/c, Z = 2, a = 5.608(1), b = 7.139(1), c = 18.575(5) Å, β = 102.60(2)° and V = 725.72(3) Å3. It occurs as yellowish brown, semi-parallel aggregates of elongated, flattened crystals up to 1 cm in length. The mineral is translucent, with vitreous lustre, good {001} cleavage and uneven fracture. Mohs' hardness is 41/2; the measured density is 3.49(1) g/cm3. The strongest five X-ray diffraction lines of the powder pattern [d in Å (1)] are: 2.898(100), 3.027(68), 2.613(26), 2.459(24), 1.853(24). An average of fifteen electron microprobe analyses of grenmarite gave SiO2 29.85, TiO2 4.51, CaO 1.84, MnO 8.25, FeO 2.01, Na2O 15.43, Y2O3 0.22, Ce2O3 0.01, ZrO2 31.63, Nb2O5 0.06, F 5.24, O ≡ F 2.21, total 96.84 wt.%. The empirical formula, based on 4 Si atoms, is (Zr1.52Mn0.46Y0.02)Σ2.00 (Zr0.55Ti0.45)Σ1.00 (Mn0.48Na0.29Fe0.23)Σ1.00 (Na3.72Ca0.26)Σ3.98 Si4O15.40F2.22.

Grenmarite is isostructural with seidozerite: its structure can be described as built up with three distinct modules connected through corner-sharing: an “octahedral” layer, an “octahedral” ribbon, and disilicate groups. The crystal structure has been refined to a final R of 0.043. The crystal chemical formula resulting from the structural refinement is: (Zr1.76Mn0.24)Σ2.00(Zr0.54Ti0.46)Σ1.00(Mn0.76Na0.24)Σ1.00(Na3.32Ca0.38Mn0.30)Σ4.00(Si2O7)2O1.96F2.04.

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